AWWA WQTC56943

Bench-scale experiments were conducted with low- and medium-pressure ultraviolet (UV) lamps to inactivate pathogenic and nonpathogenic strains of Escherichia coli and mixtures of naturally occurring heterotrophic bacteria suspended in granular-media filtered water. Prior to UV exposure, E. coli life cycle characteristics were measured to understand the effects of cell dormancy (a characteristic that may be similar to E. coli present in the environment) on inactivation kinetics. Following UV exposure, bacteria were incubated at 20oC in lighted and dark environments to simulate subsequent treated water storage in open-air and enclosed reservoirs, respectively. Approximately 9 mJ/cm2 of UV light provided 2-log inactivation of heterotrophic bacteria. However, suspensions of heterotrophic bacteria treated with 140 mJ/cm2 were able to regrow to pre-treatment levels during light or dark incubation. Incubation of UV- treated bacteria with a 2.5 mg/L chloramine residual prevented regrowth. For E. coli, resistance to UV varied significantly. To provide 2-log inactivation of all the strains studied, a dose between 0.2 and 7 mJ/cm2 was required. E. coli resistance to UV was observed to increase when DNA replication was dormant. UV-inactivated, light-incubated E. coli were able to repair within 2 hours of treatment. To provide a more complete barrier to pathogens and sustain public health protection, UV treatment should be followed with a residual disinfectant (e.g., chloramines). Includes 15 references, figures.